1. Field of the Invention
This invention relates to a semiconductor device molded in a resin and, more particularly, to a semiconductor device having an improved lead structure.
2. Description of the Related Art
A conventional semiconductor device such as the one shown in FIG. 1 is known. The device shown in FIG. 1 is formed of a semiconductor chip 1, a lead frame 2, thin metallic wires 3, and a mold resin 4. The semiconductor chip 1 has first and second surfaces 12 and 13 which are rectangular and which face in opposite directions. The lead frame 2 is formed from a metallic foil made of a Fe-Ni alloy, phosphor bronze or the like, and has a die pad 21 and a plurality of leads 22 disposed around the die pad 21. The second surface 13 of the semiconductor chip 1 is fixed on the die pad 21. Each of electrodes 11 formed on the first surface 12 of the semiconductor chip 1 is electrically connected by the thin metal wire 3 made of Au, Al or the like to an end of the corresponding lead 22 which serves as a wire bonding portion 23. As shown in FIG. 2, the semiconductor chip 1, the die pad 21, the thin metallic wires 3 and portions of the leads 22 are molded in the mold resin 4 which is, for example, an epoxy resin.
This type of semiconductor device is manufactured as described below.
As shown in FIG. 3B, the second surface 13 of the semiconductor chip 1 is first fixed by an adhesive or solder on the die pad 21 of the lead frame 2 punched to form the die pad 21 and the plurality of leads 22 as shown in FIG. 3A. Next, each of the electrodes 11 formed on the first surface 12 of the semiconductor chip 1 is electrically connected to the wire bonding portion 23 of the corresponding lead 22 by means of the thin metallic wire 3. Thereafter, the semiconductor chip 1, the die pad 21, the thin metallic wires 3 and inner lead portions 25 of the leads 22 are molded in the mold resin 4, as shown in FIG. 3C, so that these components are protected from the outside air. Next, the outer lead portions 26 of the leads 22 exposed outside the mold resin 4 are bent so that they have suitable profiles, and die pad supporting portions 24 which support the die pad 21 are cut.
Recently, the outside dimensions of molded semiconductor devices have had to be reduced, while the semiconductor chip 1 typically a dynamic RAM molded in the mold resin 4, has an increased size.
In the lead frame 2 used in the conventional semiconductor device, however, the wire bonding portion 23 formed at the inner end of each lead 22 is positioned in the vicinity of the peripheral portion of the die pad 21, as shown in FIG. 3A. Therefore, the length L of the inner lead portion 25 embedded in the mold resin 4 as shown in FIG. 2 must be reduced. A reduction in the length L of the embedded inner lead portion 25 may cause a considerable concentration of stress at the boundary between the wire bonding portion 23 of the lead 22 and the mold resin 4 when the outer lead portion 26 is bent and may reduce the strength of adhesion between the wire bonding portion 23 and the mold resin 4, thereby increasing the possibility of water entering the semiconductor device through the interface between the lead 22 and the mold resin 4. In consequence, this type of semiconductor device is lacks good reliability.